Field of the Invention
The invention relates to a system and a method for neural stimulation of a patient's hearing, such as by cochlea stimulation.
Description of Related Art
The sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous auditory prosthesis systems (e.g., cochlear implant (CI) systems) have been developed. auditory prosthesis systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
To facilitate direct stimulation of the auditory nerve fibers, a lead having an array of electrodes disposed thereon may be implanted in the cochlea of a patient. The electrodes form a number of stimulation channels through which electrical stimulation pulses may be applied directly to auditory nerves within the cochlea. An audio signal may then be presented to the patient by translating the audio signal into a number of electrical stimulation pulses and applying the stimulation pulses directly to the auditory nerve within the cochlea via one or more of the electrodes.
Typically, the audio signal, which usually is captured by a microphone, is divided into a plurality of analysis channels, each containing a frequency domain signal representative of a distinct frequency portion of the audio signal, wherein the frequency domain signal in each analysis channel may undergo signal processing, such as by applying channel-specific gain to the signals. The processed frequency domain signals are used for generating certain stimulation parameters according to which the stimulation signals in each stimulation channel is generated. The analysis channels are linked to the stimulation channels via channel mapping. The number of stimulation channels may correspond to the number of analysis channels, or there may be more stimulation channels than analysis channels, or there may be more analysis channels than stimulation channels. Various stimulation strategies are used, such as current steering stimulation (in order to stimulate a stimulation site located in between areas associated with two or more electrodes) and N-of-M stimulation (wherein stimulation current is only applied to N of M total stimulation channels during a particular stimulation frame).
An example for such a CI system with electrical cochlea stimulation is described in International Patent Application Publication WO 2011/032021 A1 and corresponding U.S. Pat. No. 8,422,706.
Human sound source localization ability is mainly based on detection of interaural time or level differences (ILD/ITD) in the signals picked up by the two ears. In addition, the pinna provides for some monaural cues due to direction and elevation dependent changes of the frequency spectrum of the sound impinging onto the pinna. In general, CI users have difficulties to localize sound sources, since the ITDs are distorted by the CI signal processing.
In case of a unilateral CI usage the auditory system receives input only from the ear provided with the CI. Therefore, in such case the localization ability is very weak, since the direction dependent sound shaping of the pinna, at best, allows only for a very rough front/back differentiation or left/right differentiation.
In case of a bimodal fitting, in which an acoustic hearing aid is worn on the contralateral ear in addition to the CI provided at the ipsilateral ear, localization ability still is quite weak. Although bimodal fittings provide binaural input to the auditory system, localization cues (ILD/ITD) are still largely absent due to the usually only little overlap of the frequency range of the electrical stimulation and the frequency range of the acoustical stimulation (typically, the hearing aid stimulates the lower frequencies up to 1-1.5 kHz, while CI stimulation does not take place at frequencies below 1 kHz due to the limited insertion depth of the CI electrode).
In case of a CI fitting with routing of a microphone signal from a microphone positioned at the contralateral ear (“CROS” design), the localization ability is more or less lost, since the head shadow cue is no longer available due to the addition of a routed contralateral microphone signal to the ipsilateral microphone signal of the CI speech processor (such CROS systems are used for overcoming the head shadow in order to improve speech understanding in cases where speech comes from the non-implanted side).
U.S. Pat. No. 8,503,704 B2 relates to a binaural CI system, wherein the localization cues are modified in the neural stimulation signals in order to enhance the localization ability of the patient; for example, a detected localization cue, such as an interaural level difference (ILD), is transposed to a lower frequency.
U.S. Pat. No. 8,526,647 B2 relates to a hearing instrument, such as a CI, wherein directional cues may be artificially generated by frequency shaping based on information from a directional system in order to improve front/rear discrimination, with one microphone being located at each ear of the user.
U.S. Pat. No. 8,285,383 B2 relates to a CI system including directional sound processing by enhancing the sensitivity of at least one associated microphone to the desired sounds and essentially treating undesired sound as noise.
U.S. Pat. No. 8,953,817 B2 relates to a binaural beamformer to be used in CI devices, comprising a microphone at each side of the head, wherein a single channel output signal is produced by modifying the signals captured by the microphones in a manner so as to insert localization cues according to the head-related transfer functions to enable to user to perceive the direction of the sound, wherein the single channel output signal is supplied to both ears. The interaural time difference may be used to cancel relatively low frequency sounds, depending on the direction of incidence, and the interaural level difference can be used to cancel high frequency sounds depending on their direction of incidence in a weighted sum configuration. The system is concerned with improving directionality—but not localization—of a binaural hearing system, wherein a Wiener filter type blocking matrix is used to filter speech signals, with the weighted sum of filtered and direct signals being produced at each ear.